Moving Parts Where You Least Expect Them…

There’s good news and bad news to report today…

We’ll start with the good news. The throttle linkage bracket is painted, installed and working:

The bad news is that I have to take part of the brake system apart again. Ugh…

As I mentioned last week, the “check brakes” idiot light in the Mustang is always illuminated. This light is intended to inform the driver that there has been a catastrophic failure in the braking system. I happen to know my brakes are fine, but sadly, the light is still indicating otherwise. Today’s plan was to find out why.

As it turns out, the Mustang actually has the two braking systems (front and rear) that are independent of one another. If a catastrophic failure happens in one, the other still functions. This is a handy safety feature.

Before the brake fluid is sent to the wheels, it passes through what’s called a distribution block. A picture of my distribution block is shown above. Never in a million years would I have expected there to be moving parts inside this thing but surprise! – there are.

Inside this distribution block is a set up that looks something like this:

You can see that the two independent systems have a piston sandwiched between them (shown above in red). In the middle of this piston is a recessed area. Inside this recessed area is another piston (shown in blue) that attaches to a switch that drives the dashboard light.

The dashboard light is wired such that it always has 12 volts supplied to the light, but under normal circumstances, there is no path to ground to complete the circuit:

If a brake failure happens in one of the two systems, the following will take place inside the distribution block:

The system in the “failed” state will not have any hydraulic pressure. The pressure in the working system will force the piston between the two chambers in the distribution block to move towards the chamber of the failed system. As a result, the piston on the switch will no longer be inside the recessed area. Instead, it will be forced up into the switch and complete the circuit to illuminate the light:

The wiring is actually slightly more complex than what I’ve shown above. There’s a second path to ground for this light as well that is actuated whenever the car is turning over. This “proves out” that the light is working by illuminating it during start up. Once the engine is no longer cranking and running, the “prove out” circuit closes and the light turns off if the system is working properly.

According to the official Ford service manual, the distribution block piston can get moved if you bleed the brakes. This makes sense-the brake system that’s being bled will have a lower pressure than the one that’s not. In that case, you’re supposed to do the following to fix the problem:

Centralizing the Pressure Differential Valve…to centralize and turn off warning light, a pressure differential or unbalance condition must be created in the opposite brake system from the one that was repaired and bled last. Turn ignition to ACC or ON position. Loosen differential valve assemply brake tube nut at the outlet port on the opposite side of the brake system that was repaired and/or bled last. Depress the brake pedal slowly to build line pressure until the pressure differential valve is moved to a centralized position and the brake warning light goes out; then, immediately tighten the outlet port tube nut to the specified torque. Check fluid level and fill to within 1/4 inch of the top. Turn ignition switch to OFF.

My wife and I tried to do a variation of that today. Sadly, we had no luck whatsoever. The light stayed on.

Then we removed the hood, the hinge and the switch and I tried to move the piston manually. All I think I accomplished was scratching the piston up a bit. It appears to be well and truly stuck.

With a fair amount of Googling, I found that it’s fairly common for the distribution block piston to get stuck. Many people simply unplug the switch and leave it at that. I was very very tempted to do the same – I really don’t want to take the brake system apart again. However, I’m trying to do this restoration right and if that means I have to take that stupid part of the again, well then so be it. Thankfully, I still have the tools I purchased last time I had to futz with the distribution block. I don’t think I’ll need to remove the brake booster or the master brake cylinder.

The purpose of taking the distribution block off will be to break the piston loose and then clean out the innards and replace the O-rings that keep the brake fluid from contaminating the area where the piston lives. They do make rebuild kits for both the distribution block and the proportioning valve and I ordered both this evening.

I can’t say I’m terribly pleased with how this turned out, but life goes on. I get more practice with brakes.

yea…